1. Field of the Invention
The present invention relates to an engine speed control apparatus and an engine speed control method for controlling an engine speed. Further, the present invention relates to an engine system having such an engine speed control apparatus, and also relates to a vehicle and an engine generator each having such an engine system.
2. Description of Related Art
The engine speed in an idling state is susceptible to influences of environmental conditions such as atmosphere and humidity, and is therefore unstable. Accordingly, an ISC (Idle Speed Control) control is conducted, at idling time, on a vehicle having an engine mounted thereon, particularly a two-wheeled motor vehicle.
A known ISC-control is disclosed in the Japanese Patent Laid-Open Publication (KOKAI) No. 5-263703. This prior art uses a throttle sensor for detecting the opening degree of a throttle valve (throttle opening degree) disposed in the main air intake passage of the engine. By controlling, to a target opening degree, the throttle opening degree detected by this throttle sensor, the idling engine speed is controlled.
In the idling engine speed zone, the engine speed is significantly changed by small changes in an intake air amount. It is therefore necessary to detect the throttle opening degree with high resolution (the throttle opening degree of about 0.02°) such that the throttle opening degree is precisely controlled.
For example, the throttle sensor has linear characteristics such that the output value thereof is 0V when the throttle opening degree is 0° and the output valve is 5V when the throttle opening degree is 90°.
When the output signal of the throttle sensor is analog/digital converted with an 8-bit A/D converter, for example, the throttle opening degree per bit is about 0.35°, thus failing to obtain sufficient resolution.
Accordingly, in the prior art of the Japanese Patent Laid-Open Publication (KOKAI) No. 5-263703, an output signal of a throttle sensor is amplified by an amplifier and then input into an A/D converter to improve the throttle opening degree detection resolution in the low opening degree zone.
However, this prior art requires an amplifier for enhancing the throttle opening degree detecting resolution, which disadvantageously increases the cost.